The present invention pertains to a method for heat-treating metallic workpieces, in which a flow of cooling gas is generated in a vacuum furnace by a fan in order to quench the workpieces, with the fan being driven by a rotary current motor that is operated with a predetermined supply voltage above a minimum pressure in the vacuum furnace, which pressure is determined with regard to the motor power of the rotary current motor.
In the heat treatment of metallic workpieces, e.g., hardening, tempering or annealing, vacuum furnaces are increasingly utilized. The workpieces are cooled in these vacuum furnaces by a gaseous medium, e.g., nitrogen, after being heated. In comparison to conventional oil bath quenching or salt bath quenching methods, such a gas quenching provides the advantage that no contamination of the workpieces occurs, i.e., costly cleaning measures are eliminated. In order to achieve cooling effects similar to those of the oil bath quenching or salt bath quenching method during the gas quenching, it is known to provide high cooling gas pressures that ensure the desired heat transfer due to the increased gas density associated therewith. However, high cooling gas pressures require complicated safety measures, with the time required for flooding or evacuating the vacuum furnace also being relatively long.
Another disadvantage that occurs during high-pressure gas quenching can be seen in the fact that the fan used for generating the flow of cooling gas in the vacuum furnace requires a comparatively high shaft output so as to ensure the required cooling gas speed for the load moments occurring at high pressures. A high shaft output also makes it necessary to achieve a high motor power of the electric motor driving the fan. Consequently, this electric motor is usually realized in of the form of a rotary current motor with a rated power of, for example, 220 kW. A rated motor power of 220 kW results in a rated motor current of 400 A at a supply voltage of approximately 400 V. When the fan is started, a starting current of 3600 A is created due to the surges which occur during this process and which usually amount up to nine times the rated motor current under standard conditions of the cooling gas.
High currents of this type frequently result in network interruptions and high wear, primarily at the connecting points. This is prevented by utilizing starting devices for realizing a so-called soft start of the rotary current motor. This is achieved by limiting the starting current, e.g., to five times or six times the rated motor current. However, such starting devices are associated with higher costs and consequently not considered satisfactory with respect to economic considerations.
Although the soft start of the electric motor driving the fan makes it possible to quench the workpieces to be treated at low furnace pressures, i.e., during the flooding of the vacuum furnace, the beginning of the quenching process is subject to a lower limit with respect to time. This can be attributed to the fact that the vacuum furnace needs to be flooded to a minimum pressure which is defined with regard to the supply voltage of the rotary current motor before the fan can be started. This measure serves for preventing the occurrence of, for example, flashovers that result in insulation damages. For rotary current motors with a motor supply voltage of 400 V, the minimum pressure which can be determined with the aid of so-called Paschen curves usually lies at approximately 750 mbar.
Since the fan can only be started once the minimum pressure during the flooding of the vacuum furnace with a cooling gas is reached, the quenching time and consequently the attainable quenching effect are disadvantageously influenced due to the unavoidable starting time of the fan.
An object of the invention is to develop a method for heat-treating metallic workpieces in such a way that an improved quenching effect can be achieved in a simple and inexpensive fashion.
The above and other objects of the invention can be attained due to the fact that the fan is started at a pressure in the vacuum furnace which is lower than the minimum pressure that can be selected, for example, from the range of 500-1200 mbar with the rotary current motor being operated with a second, lower supply of voltage until the minimum pressure in the vacuum furnace is reached.
Such a method makes it possible to achieve an improved quenching effect. The primary cause for this is that shorter quenching times which allow a higher variability with respect to the desired quenching behavior for the respective workpieces to be treated can be achieved due to the start of the fan at a pressure in the vacuum furnace which is lower than the minimum pressure.
A feature of the invention is that a start of the fan at pressures below the minimum pressure is possible without risking flashovers if the rotary current motor is operated with a lower supply voltage than required for the shaft output of the fan necessary for the stipulated cooling gas speed. The reduced supply voltage also reduces the starting current, i.e., a starting device that makes it possible to realize a soft start can be eliminated. Although the lower supply voltage also reduces the motor power, the motor power suffices for starting the fan due to the low pressure in the vacuum furnace and the low density of the cooling gas associated therewith.
Once the minimum pressure in the vacuum furnace is reached, the fan is operated with the higher supply voltage. Since the fan already rotates with its nominal speed at this time, the shaft output required for quenching the workpieces is immediately available once the change-over to the higher supply voltage takes place, namely without impairing the quenching effect due to the time loss caused by the starting of the fan as is the case with the state of the art. In this respect, it is particularly advantageous for kinetic energy to be already stored in the fan before the minimum pressure in the vacuum furnace is reached due to the rotation of the fan, with said kinetic energy manifesting itself in the form of a flywheel effect when the change-over to the higher supply voltage takes place. Due to the lower starting currents, the method according to the invention also contributes to a more favorable current consumption with respect to economic considerations and makes it possible to eliminate very high quenching pressures that are difficult to realize while still achieving a comparable quenching effect.
It is particularly advantageous if the supply voltage is applied to the rotary current motor and decreased from a higher to a lower supply voltage and increased vice versa by a transformer. The voltage transformation by means of a transformer is comparatively inexpensive and makes it possible to easily retrofit existing heat treatment systems such that the method according to the invention can be carried out. For the same purpose, the invention proposes that the rotary current motor be operated with a supply voltage of approximately 400 V above the minimum pressure and with a supply voltage of approximately 230 V below the minimum pressure.
According to one preferred additional development of the invention, the supply voltage applied to the rotary current motor is changed depending on the pressure in the vacuum furnace and/or the intensity of the current flowing through the rotary current motor so as to ensure that the method can be carried out as easily as possible and automated. In an additional development of the invention, a minimum pressure of 750 mbar is proposed such that the motor power of the most common rotary current motors for fans used in vacuum furnaces is taken into consideration.
In order to allow the utilization of powerful rotary current motors, the rotary current motor is cooled with water according to another characteristic of the invention. A simple control of the cooling gas flow can be achieved by varying the speed of the fan above the minimum pressure depending on the desired cooling gas speed. The invention also proposes that the fan be operated at pressures in the vacuum furnace up to 40 bar so as to ensure cooling gas pressures that correspond to the respective requirements while still achieving a sufficient quenching effect.
The aforementioned amendments do not add any new matter to the application. The new paragraph on page 3 of the specification reiterates the minimum pressure range detailed in the originally filed claims. The new paragraph on page 5 details that the fan can be operated at pressures in the vacuum furnace up to 40 bar, which is also supported by the originally filed claims.